Custom Tool Parameter

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1

I want to add a tool (custom function in retell) which is a basic weather forecast api request.
It is a GET request, therefore in the UI, i CAN NOT choose if the “parameters” should be description (filled by the model) or value (constant).

I am asking your Helper AI on the official website and it insists that there IS a toggle to switch this query type in the GET requests but there is not.

Typing {{q}} to tell the model it needs to fill this itself doesn’t work either. Model is calling the tool with no parameters, i can not navigate the model properly

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2

You’re correct—GET requests in Retell AI custom functions do not support the description/const value toggle for query parameters(1). That toggle is only available for POST, PATCH, and PUT requests(1).

For GET requests with dynamic query parameters, you need to use the Extract Dynamic Variable function first to capture the value from the conversation, then reference it in your query parameters using {{variable_name}}(2)(3)(4).

Here’s the workflow:

  1. Extract the location using the Extract Dynamic Variable function before calling your weather API(2)(3)
  2. Set up your query parameter in the custom function with double curly braces: {{city}} or {{location}}(5)(2)(4)
  3. Ensure the variable has a value before the tool is called—otherwise it will pass the literal text {{q}} instead of the extracted value(2)(4)

Important notes from community issues:

  • Variables must use double curly braces {{variable_name}}, not single {variable_name}(5)
  • The Extract Dynamic Variable function must run before your custom function call(2)(3)
  • The variable name in your extraction must exactly match what’s in your URL(2)
  • Test calls via web/chat may not work properly if the variable isn’t set—phone calls are more reliable for testing(4)

If you’re still having issues after setting up extraction, check your call logs to verify the variable has a value in collected_dynamic_variables before the tool is invoked(2).